The present invention relates to rolling bearings, particularly to rolling bearings suitable for use with a cleaning/chemical polishing apparatus used in environments in which they are brought into contact with corrosive fluoride compounds for cleaning and chemical polishing.
Generally, in wafer cleaning and chemical polishing steps in semiconductor manufacturing facilities, corrosive fluoride compounds such as hydrofluoric acid are used as cleaning agents or surface treatment agents. For example, in a Chemical Mechanical Polishing (CMP) apparatus, wafer surfaces are cleaned and polished by a rotary brush while showering hydrofluoric acid on wafers.
Rolling bearings used in or with the carrying unit and rotary brush of such a cleaning apparatus are made of a corrosion-resistant material. For example, bearing rings, rolling elements and retainers made of stainless steel are adopted (JP patent publication 2003-139147A).
On the other hand, in automotive electric accessories, for which improved heat resistance is required, fluorine lubricating grease is sealed in rolling bearings used in such accessories and contact seals made of fluorine rubber for sealing the gap between inner and outer rings are adopted (JP patent publication 2004-150477A).
The above-mentioned heat-resistant fluorine lubricating grease is made by mixing perfluoropolyether oil with a thickening agent formed of fluorine resin particles such as polytetrafluoroethylene (hereinafter PTFE) for thickening.
Such fluorine lubricating greases are stable to various kinds of solvents. Thus they are suitable as lubricating greases for rolling bearings used in a cleaning apparatus.
But if such fluorine lubricating grease is sealed in conventional rolling bearings having bearing rings, rolling elements, and a retainer that are each made of stainless steel, perfluoropolyether (PFPE) mixed therein as a base oil reacts with stainless steel of the metal material in a boundary lubrication state. As a result, the main chain of PFPE is cut at positions of the ether groups and a fluoride (RF—COF) having an acyl group is produced. The fluoride is a kind of Lewis acid having extremely high corrosivity and acts on PFPE, which is a base oil, as a decomposition catalyst, so that the fluorine lubricating grease will be rapidly decomposed and flow away, thus causing metal abrasion of the rolling bearing.
By such corrosion mechanism, rolling bearings made of stainless steel and having fluorine lubricating grease sealed therein tend to have a short component life due to the rapid progression of internal wear.
Also, even rolling bearings made of stainless steel which do not contain fluorine lubricating grease corrode and produce metallic abrasion powder in use conditions in which they are exposed to hydrofluoric acid as a cleaning or polishing agent. The metal abrasion powder infiltrates into rolling portions of the bearing, causing boundary lubrication and thus abnormal bearing wear.
An object of the present invention is to provide a rolling bearing made of stainless steel which, in highly corrosive use conditions where hydrofluoric acid or fluorine grease is present, can suppress boundary lubrication resulting from infiltration of metal corrosion powder into rolling surface of the bearing and rapid progression of internal wear, and which is highly durable and has a long life even if used in metal corrosive environments.